This invention relates to a chain drive for a transfer machine which permits each workpiece to be smoothly transferred on a chain-driven pallet at a constant speed from one machine work station to another.
Transfer machines of automated assembly lines, conveyor lines and other such installations use chain drives as a means for moving work-carrying pallets through successive work stations. However, the conventional chain drives involve difficulties in smoothly and precisely transferring the pallets. This is particularly true with the large-size transfer machines of recent designs which demand increased power for the chain driving. In those machines, a plurality of chains, usually two in parallel, are driven at the same speed in a given direction so as to transfer a pallet bridging over the chains. It has been found that the individual chains being driven in the same way actually differ slightly in speed, the differential increasing in proportion to the period of service. Understandably, equality of the individual chain speeds is essential for smooth pallet conveying. Factors responsible for the differentiation of the chain speeds are as follows. At each bend or corner of the track formed by the chains of a transfer machine, sprockets must be provided to ensure the smooth movement of the chains. In practice, as shown in FIG. 1, those sprockets at the corner are utilized to drive the chains, the inner sprocket 21 driving the inner chain 23, and the outer sprocket 22 the outer chain 24. As shown, the inner chain provides more wrap on the inner sprocket 21 than does the outer chain on the outer sprocket 22, with a longer distance of engagement in the arc of contact therebetween than that between the outer couple. This tends to induce the stick-slip of the outer chain 24, a phenomenon in which the chain speed becomes too slow and too fast alternately and intermittently. Should the phenomenon occur, the slight difference resulting between the speeds of the individual chains will make the smooth pallet transfer no longer possible. Another adverse factor is the wear with use of the rollers and pins of the chains and that of the sprocket teeth. This will increase the chances of stick-slipping of the outer chain, with a greater contact pressure because of the less wrap on the outer sprocket than that of the inner chain on the inner sprocket. Especially on installations after extended service periods, the conveying conditions of the inner and outer chains for driving the pallets thereon will become unbalanced, making the smooth pallet transfer infeasible.